The present invention relates to the manufacture of rotary sheet dies used for precise cutting of patterns in gasket material, paper, et cetera. More specifically, the present invention relates to the electrical discharge machining (EDM) of rotary sheet dies which are used for cutting patterns in such materials.
It has long been recognized that the use of rotary or cylindrical dies to cut blanks or patterns from sheets or webs of material such as paper, cardboard, plastic film, et cetera, is efficient and cost effective. A description of one method of using cylindrical dies in that fashion can be found in U.S. Pat. No. 4,608,895. Because of the accuracy and longevity required of such cylindrical dies, a number of machines and methods have been developed in the prior art to manufacture them. Some cylindrical cutting dies have been manufactured using a milling machine for cutting the die pattern. However, it has been found that the use of EDM technology to erode the die pattern on the die cylinder can provide a more efficient and accurate way of producing a die having a complex pattern. Examples of such methods and apparatus can be found in U.S. Pat. Nos. 3,542,993; 3,796,851; 3,890,481; and 3,952,179. However, these prior art devices and methods suffer from a number of significant problems. For example, those that use a cylindrical EDM electrode, as in U.S. Pat. No. 3,796,851, are more expensive because of the cost and difficulty in manufacturing the electrode. In addition, such an electrode is more difficult to machine, as compared to flat electrodes, and requires frequent adjustment of the position of the electrode and/or die cylinder to maintain proper arc spacing and alignment. Also, sophisticated timing devices must be used in order to properly synchronize the surface speeds of the cylindrical die work piece and the electrode.
The use of a flat electrode with a cylindrical die work piece, as in U.S. Pat. No. 3,890,481, solves some of these problems but creates more of its own. In a horizontal die cutting device such as that disclosed in the '481 patent, sediment and material eroded from the die cylinder is naturally trapped in the V-shaped grooves of the electrode, making flushing extremely difficult. Clean flushing is, of course, critical in order to provide a uniform spark gap which is very difficult to do if the dielectric fluid becomes contaminated with eroded sediment. In addition, it is very difficult to maintain proper orientation of the die cylinder as it moves horizontally across the fixed electrode matrix, as it has a tendency to skew as it moves. This results, of course, in inaccurate and imprecise cutting of the die pattern on the cylinder. Another problem with the prior art methods of manufacturing rotary cutting dies is the bulk and expense involved in using solid steel cylinders as the die work piece.
Although it is recognized in the prior art that the use of rotary dies made of thin steel sheets would solve some of these problems, no one has been able to successfully use EDM technology to manufacture rotary sheet dies that are usable without extensive hand finishing. Specifically, no prior art method of manufacturing rotary sheet dies has solved the problem of how to firmly and accurately hold the sheet in position while it is being electrically eroded, because of the effects of heat and mechanical stress.
What is needed, then, is a machine and method of manufacturing rotary sheet dies that employs fast and efficient EDM methods of cutting the dies and that results in a die that is cut with a precise and accurate pattern.